The present invention is concerned with novel blast cleaning components, including a novel media valve to control the amount of abrasive media directed to the compressed air stream, a novel pressurization system which reduces the contact of moisture with the particulate abrasive contained in a media supply pot and a novel blast nozzle which reduces electric spark discharging during the blast cleaning operation.
Standard sand blasting equipment consists of a pressure vessel or supply pot to hold particles of a blasting medium such as sand, a source of compressed air connected to the supply pot via a conveying hose and a means of metering the blasting medium from the supply pot, which operates at a pressure that is the same or slightly higher than the conveying hose pressure. The sand/compressed air mixture is transported to a nozzle where the sand particles are accelerated and directed toward a workpiece. Flow rates of the sand or other blast media are determined by the size of the equipment. Commercially available sand blasting apparatus typically employ media flow rates of 10-20 pounds per minute. About 0.5 to 1 pound of sand are used typically with about 1.0 pound of air, thus yielding a ratio of 0.5 to 1.0.
When it is required to remove coatings such as paint or to clean relatively soft surfaces such as aluminum, magnesium, plastic composites and the like, or to avoid surface alteration of even hard materials such as stainless steel, less aggressive abrasives, including inorganic salts such as sodium chloride and sodium bicarbonate, can be used in place of sand in conventional sand blasting equipment. The media flow rate used for the less aggressive abrasives is substantially less than that used for sand, and has been determined to be from about 0.5 to about 10.0 pounds per minute, using similar equipment. The lower flow rates require a much lower media to air ratio, in the range of about 0.05 to 0.5.
However, difficulties are encountered in maintaining continuous flow at these low flow rates when conventional sand blasting equipment is employed. The fine particles of an abrasive media such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, the bicarbonate media particles tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. Flow aids such as hydrophobic silica have been added to the bicarbonate in an effort to improve the flow, but maintaining a substantially uniform flow of bicarbonate material to the blast nozzle has been difficult to achieve. Non-uniform flow of the blast media leads to erratic performance, which in turn results in increased cleaning time and even to damage of somewhat delicate surfaces.
Commonly assigned U.S. Pat. Nos. 5,081,799 and 5,083,402 disclose a modification of conventional blasting apparatus by providing a separate source of line air to the supply pot through a pressure regulator to provide a greater pressure in the supply pot than is provided to the conveying hose. This differential pressure is maintained by an orifice having a predetermined area and situated between the supply pot and the conveying hose. The orifice provides an exit for the blast media and a relatively small quantity of air from the supply pot to the conveying hose, and ultimately to the nozzle and finally the workpiece. The differential air pressure, typically operating between 1.0 and 5.0 psi with an orifice having an appropriate area, yields acceptable media flow rates in a controlled manner. The entire contents of U.S. Pat. Nos. 5,081,799 and 5,083,402 are herein incorporated by reference.
A media metering and dispensing valve which meters and dispenses the abrasive from the supply pot through the orifice and to the conveying hose carrying the compressed air stream typically operates automatically whenever the compressed air is applied to the blast hose to begin the abrasive blasting operation. The media valve for use in the afore-mentioned metering and dispensing process as disclosed in U.S. Pat. Nos. 5,081,799 and 5,083,402 is characterized as a Thompson valve and is described in detail in U.S. Pat. No. 3,476,440, the contents of which are herein incorporated by reference. The Thompson valve includes a metering valve stem which blocks the outlet of a discharge tube disposed between the supply pot and an air flow tube which is secured to and carries the compressed air to the conveying hose. When the blast nozzle is activated, the valve stem is lifted from the valve seat of the Thompson valve and allows a controlled amount of media to flow through the outlet of the discharge tube into the air flow tube. The valve as disclosed in U.S. Pat. No. 3,476,440 has been improved by placing the valve stem within a control sleeve which contains a plurality of orifices having different sizes, one of which can be placed in communication with the outlet of the discharge tube and the air flow tube. When the valve stem is seated within the valve body and closed, the orifice in the control sleeve is blocked such that media cannot flow from the discharge tube through the orifice in the media control sleeve and then into air flow tube. Upon operation of the blast nozzle, the valve stem is pulled away from the orifice to allow the media flow from the pot to the discharge tube, through the orifice and into the air flow tube.
The plurality of orifices provides another means of controlling the amount of media flowing from the supply pot into the compressed air stream and into the blast nozzle apparatus. Unfortunately, to change the orifice which is in alignment with the media discharge tube and the air flow tube or to clean out a plugged orifice in the Thompson valves now on the commercial market, it is required that the valve body holding the stem be taken apart, the valve sleeve taken out, rotated, placed back in its slot and the valve body then restructured. Obviously such disassembly and reassembly is cumbersome and certainly does not allow for efficient blast cleaning on the job site. Accordingly, it would be useful to provide a media control valve which can be more readily and easily changed to control the media flow from the supply pot to the air flow tube.
As briefly discussed above, moisture is often added to the media in the supply pot during pressurization. During pressurization of the supply pot, compressed air enters the media supply pot through a pop-up tube after the abrasive media has been fully loaded into the pot. Incoming air causes a pop-up valve slidably engaged in the pop-up tube to rise and seal off the media supply opening in the pot allowing pressurization of the pot and activation of the differential pressure media metering system described previously. Unfortunately, moisture accumulates in the air supply line to the supply pot and upon the initial pressurization of the media supply pot, the compressed air carries the collected pool of moisture up the pop-up tube and into the media pot moistening the media and causing portions of the particulate media to agglomerate. The agglomerated media is not readily free-flowing which often causes a non-uniform media flow from the pot. The problem of moisture is exacerbated since the initial air expands rapidly causing the air to cool which consequently causes precipitation of the trapped moisture from the air onto the particulate media. It would be worthwhile to provide a means to supply compressed air to the media supply pot for the differential pressure metering system which supply means would eliminate the problem of entrained moisture within the compressed air from leaving the pop-up tube and falling onto the particulate abrasive media in the supply pot.
Still another problem during blast cleaning often exists with the blast nozzle itself. The blast nozzles most used to convey the abrasive media such as sand, sodium bicarbonate, etc. are venturi-type nozzles which contain a converging inlet section, an orifice and an expanding outlet section which accelerates the blast media entrained in the compressed air stream to the outlet of the nozzle. If sand or other hard abrasive is utilized, the blast nozzles are often formed of a hard ceramic. A particularly useful ceramic is a reaction-bonded silicon nitride. Nozzles formed from this ceramic material are described in copending, commonly assigned PCT Application PCT/US93/09409, filed Oct. 7, 1993, the contents of which are herein incorporated by reference. The hard ceramic material which forms the nozzle is relatively brittle and is thus encapsulated in a resin such as a polyurethane protective jacket. Both the ceramic and resinous jacket are poor conductors of electricity. As the fast moving abrasive particles slide along the interior surfaces of the blast nozzle, static electricity is generated. The static electrical charges are commonly dissipated by bonding, i.e., electrically connecting the nozzle to the blast machine and grounding the blast machine to the workpiece. Bonding and grounding the blast nozzle provides an electrical path to dissipate the static charges to ground. Unfortunately, the ceramic nozzles being constructed of poor electrically conductive materials, are essentially electrically insulated from ground and accordingly, static electric charges accumulate on the nozzle and discharge periodically in the form of static electric sparks and nuisance shocks to the operator. These static electric sparks are undesirable as they can ignite flammable vapors, gas and dusts causing explosions.
Disclosed in afore-mentioned PCT/US93/09409 is a blast nozzle configured to accelerate the blast media, in particular, sodium bicarbonate, to speeds in excess of those generated in conventional blast nozzles. The combination of higher abrasive media speeds and nonconductible materials which form the blast nozzle amplifies the generation and accumulation of static electricity on the nozzle and consequently the electric discharge problem common in any air driven blasting system. Moreover, dry ambient air common in winter months produces even greater amounts of static charges. Accordingly, it would be worthwhile to form a high performance blast nozzle of a hard ceramic material and provide an efficient means for electrically grounding the nozzle to eliminate the build up of static electric charges and the disadvantageous sparking that results from such static charge accumulation.